Cold cathode fluorescent lamps (CCFLs) are used as the light source of a backlight system in an LCD panel. These CCFLs are driven by a drive circuit called an inverter. Because of technique progresses and consumer demands, the size of LCD panels increases continually. A single lamp can't meet the requirement for illumination of LCD panel. Two or more lamps are required instead.
As shown in FIG. 1, a primary coil 42 and a secondary coil 43 are both wound around a central column 401 of a transformer 40 used in a conventional multi-lamp drive device. The primary coil 42 is connected with a drive circuit 47. When the drive circuit 47 generates an excitation power source, an excitation current will flow on the primary coil 42 to produce magnetic flux in the central column. The magnetic flux flows through a first side column 402 and a second side column 403 and then back to the central column 401. The magnetic flux can thus be coupled with the secondary coil 43 to generate an induced voltage for driving CCFLs 46 connected to the secondary coil 43 to be on. Ballast components 48 having a high impedance are connected with the CCFLs 46 to balance the currents flowing through the CCFLs 46. Balanced inductors 43 can be used to compensate the capacitance impedance of the CCFLs 46 for balance of the output powers.
Because the primary coil 42 and the secondary coil 43 of the transformer 40 are both wound around the central column 401, they use the same magnetic circuit (the central column 401) to cause increase of the mutual inductance. When the transformer 40 drives multiple lamps, a very large load current will be produced on the secondary coil 43. This load current will induce a very large counter magnetomotive force to affect the power conversion action of the primary coil 42 and also generate large heat on the primary coil 42. If the secondary coil 43 is short circuited due to some factors, the drive circuit 47 and the primary coil 42 will be burned out.
As shown in FIG. 2, a primary coil 52 and a secondary coil 54 are both wound around a first side column 501 and a second side column 503 of a transformer 50 used in another conventional multi-lamp drive device, respectively. When the primary coil 52 accepts an excitation power source from a drive circuit 47, magnetic flux will be generated in the first side column 501 and flows to the second side column 503 and then back to the first side column 501. The magnetic flux can thus be coupled with the secondary coil 54 to generate an induced voltage for driving CCFLs 46 connected to the secondary coil 54 to be on. Ballast components 48 having a high impedance are connected with the CCFLs 46 to balance the currents flowing through the CCFLs 46. Balanced inductors 43 can be used to compensate the capacitance impedance of the CCFLs 46 for balance of the output powers.
Because the primary coil 52 and the secondary coil 54 of the transformer 50 are wound around the first side column 501 and the second side column 503, respectively, they also use the same magnetic circuit to cause increase of the mutual inductance. When the transformer 50 drives multiple lamps, a very large load current will be produced on the secondary coil 54. This load current will induce a very large counter magnetomotive force to affect the power conversion action of the primary coil 52 and also generate large heat on the primary coil 52. If the secondary coil 54 is short circuited due to some factors, the drive circuit 47 and the primary coil 52 will be burned out.
Along with increase of the number of lamps, the required power rises to increase burdens to the drive circuit and the transformer. Large heat will thus be generated by the transformer to affect its function. Moreover, because the primary coil is subject to the interference from the secondary coil, normal function of the transformer will be affected. If the secondary coil of the transformer is short circuited due to some factors, the primary coil will be burned out due to a very large counter magnetomotive force induced by the short-circuit current.
Accordingly, the present invention aims to propose a multi-lamp drive device, which reduces the heat generated by a transformer due to a too large load current and also accomplishes the protection effect for the transformer during short circuit by arranging positions of winding coils of the transformer when driving multiple lamps. Moreover, the primary coil won't be affected due to load change of the secondary coil.